From: gtaylor@tnetconsulting.net   
      
   On 1/26/20 1:02 PM, Joe Chisolm wrote:   
   > See: https://en.wikipedia.org/wiki/Tip_and_ring on polarity.  It is   
   > to help with corrosion.   
      
   Thank you.   
      
   > The -48v migrated out of the telco world into the server world.   
   > Not sure why but I suspect as computers moved into the CO offices   
   > they wanted to use the battery supply.   
      
   This is my understanding.   
      
   > Data centers are moving away from the old -48V because of the   
   > inefficiencies and cabling cost.   
      
   This is my experience too.  Hence why the -48 VDC supplies and equipment   
   that I've worked with are usually much smaller and directly associated   
   with each other.  I.e. -48 VDC supply to power a piece of equipment   
   being tested in the lab.  Usually said equipment would then be deployed   
   to partners around the world that had a -48 VDC plant and wanted our   
   equipment.   
      
   > 480 3 phase to PDUs that then provide 208 or 120 to the racks is more   
   > and more common.  Better efficiency and less cost.   
      
   This is exactly what we have at my office.  13 thousand and something   
   volts 3ɸ to the property.  13, gets stepped down to 480 VAC   
   just inside the building.  Then 480 VAC is distributed to the PDUs on   
   the DC floor.  The PDUs step down the 480 VAC to 120 VAC.  The two or   
   three ɸs of 120 VAC is then distributed to cabinets.  The Cabinet   
   Distribution Units then distribute ɸ-to-ɸ of 208 VAC to equipment.  Some   
   CDUs are 1ɸ with two hots and a neutral / ground.  (I'm not sure if the   
   3rd wire is actually neutral or ground.)  Other CDUs are 3ɸ with three   
   different sets of ɸ-to-ɸ; Black-to-Red / Red-to-Blue / Blue-to-Black.   
      
   I'm re-using the numbers that have been used in this discussion.  The   
   quick math shows that 110 VAC ɸ-to-neutral results in 209 VAC ɸ-to-ɸ.   
   120 VAC ɸ-to-neutral results in 228 VAC ɸ-to-ɸ.   
      
   ɸ1 = 0°   
   ɸ2 = 120°   
   ɸ3 = 240°   
      
   V = 120 VAC   
                  ____________________________________________________________   
   ɸ-to-ɸ VAC = √ (sin(ɸ1) × V + sin(ɸ2) × V)² + (cos(ɸ1) × V +   
   cos(ɸ2) × V)²   
      
   This brings up another long standing question of what is the typical   
   voltage in the U.S.A.  110 / 120 / 125 / ???  But I digress.   
      
      
      
   --   
   Grant. . . .   
   unix || die   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)